1. Field of the Invention
This invention relates to an aqueous slurry composition and to a method of making such a composition.
2. Brief Description of the Prior Art
Aqueous particulate slurries are commonly used or encountered in many industries including the petroleum, pipeline, construction and cleaning industries. Slurries are mixtures normally consisting of particulates and an aqueous liquid and play an essential role in many industrial operations. For example, slurries are used when transporting particulates above ground, from the surface to a subterranean formation or from a subterranean formation to the surface. The most commonly used particulates include sand, ceramic particles, carbonate particles, glass spheres, bauxite (aluminum oxide), resin coated particulates and coal particulates The particulate sizes normally range from about 10 to about 100 US mesh, and the particles have densities significantly higher than the density of water. For example, the density of sand is at about 2.6 g/cm3 while the density of water is 1 g/cm3. Sand is by far the most commonly used particulate.
To make relatively stable slurry, particulates must be suspended in a liquid medium for a lengthy period at static or/and dynamic conditions. Convention wisdom tells us that the viscosity or viscoelasticity of the liquid medium must be sufficiently high to be able to suspend particulates. The most commonly used method for increasing viscosity or viscoelasticity of a liquid medium is by adding a viscosifier, for example, a natural or synthetic polymer or a viscoelastic surfactant to the liquid medium. It is not unusual that a polymer is used with a foaming agent in order to take advantage of both viscoelastic and foaming properties. However, the use of polymers in slurries increases cost and results in operational difficulties. In particular applications, for example, hydraulic fracturing of subterranean formations, the use of polymers in slurry impedes oil and gas production due to large amounts of residue left in the formation. As for viscoelastic surfactants, although they have fewer residues compared to normal polymers, their cost is usually much higher. In many other applications such as gravel-pack, well completion and sand transportation through pipelines, it is highly desirable to make stable particulate slurry without using a viscosifier.
Hydraulic fracturing operations are used extensively in the petroleum industry to enhance oil and gas production. In hydraulic fracturing, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate fracturing, which increases oil and gas production. Frequently, particulates, called proppants, are suspended in a fracturing fluid and transported into a fracture as slurry. Proppants include sands, ceramic particulates, glass spheres, bauxite particulates, resin coated sands and other particulates known in the industry. Among them sand is by far the most commonly used proppant. Fracturing fluids in common use include water-based as well as hydrocarbon-based fluids. In water-based fracturing fluids, a polymer or viscoelastic surfactant is normally employed to increase the viscoelasticity of the fluid. In most case the viscoelastic property of the fluids is essential for transporting proppants deep into a formation. At the last stage of the fracturing treatment, fracturing fluid flows back to the surface and the proppants are left in the fracture forming a proppant pack to prevent the fracture from closing after pressure is released. A proppant-filled fracture provides a highly conductive channel that allows oil and/or gas to seep through more efficiently to the wellbore. The conductivity of the proppant pack plays a dominant role in production enhancement. Polymer residues from fracturing fluids are known to greatly reduce the conductivity of the proppant pack. Compared to polymeric viscosifiers, viscoelastic surfactants cause less damage to formations and proppant packs. However, they are much more expensive. Accordingly, a need exists for a composition for efficiently transporting proppants deep into a formation at low cost while at the same time causing little damage to the formation and proppant pack. Grain size, concentration, and the packing pattern of proppants are also important factors in determining the conductivity. Despite extensive research in recent years, limited progress has been achieved to maximize the conductivity of a proppant pack in a fracture. Therefore, a need exists for making a composition for use in a proppant pack with improved conductivity.
Proppant flowback after fracturing treatments has long been plaguing the petroleum industry. Flowback reduces the amount of proppants in the formation leading to a less conductive fracture. As disclosed, for example in U.S. Pat. No. 6,047,772, various methods have been tried to solve the flowback problem. In one method, resins are used to coat the proppant and make them very tacky. In doing so, the proppant grains tend to agglomerate reducing flowback. This method is not only expensive, but the tacky resins introduced in the proppant pack tend to reduce its conductivity. Therefore, there is a need for a composition and method for making slurry, which can form a stable proppant pack, which resists proppant flowback while at the same time has a high conductivity.
When drilling subterranean formations for oil and gas, aqueous-based drilling fluids are normally used. During drilling large amounts of particles, called cuttings are generated. Cuttings have different sizes ranging from fines to pebbles. The drilling fluid is circulated through the wellbore to make slurry with the cuttings in situ and transports them out of wellbore. In most cases, polymers as well as clays are added to the drilling fluids to increase their viscosity/viscoelasticity in order to transport the cuttings efficiently. However, polymers as well as clay fines, can easily penetrate pores or thin fractures in a formation and reduce formation permeability significantly, especially near a wellbore. Reduced formation permeability impedes oil and/or gas production. Therefore it is highly desirable to provide a drilling fluid that can make stable slurry in situ with the cuttings and transport them out of the wellbore, while causing little formation damage.
The escalating price of oil and its alarming depletion rate have caused people to consider using coal as an oil substitute. Several factors have slowed the substitution of coal for oil. One factor is the difficulty in transporting coal cost-effectively over long distance through pipelines. It is therefore highly desirable to provide a composition for making coal slurry which is stable, highly fluid and cost-effective to transport.
In oil sand operation massive amount of sands are left after oil is stripped from the sand surface. Finding a more cost effective way to transport sands efficiently over distance through pipelines has long been required in the industry. Thus, a composition and a method for making stable and highly fluid sand slurries at low cost would be quite useful.
The object of the present invention is to meet the above defined needs by providing an aqueous slurry composition, which can be used in a stable, highly conductive proppant pack, for efficiently transporting proppants into a subterranean formation, and for use in transporting cuttings, coal and sand.